When an accident causes Dr. McCoy to go temporarily insane, he escapes to a strange planet. There, the search party discovers a device left by a superior, vanished civilization, a time portal that plays the history of Earth for them - but then Bones jumps through it into the past, causing a change in history important enough to make the Enterprise vanish. Kirk and Spock, who fortunately made a tricorder recording, must attempt to go through to just before McCoy's arrival and stop him from changing history in the United States during the Great Depression, where they have no advanced technology available.
When a drug-crazed Dr. McCoy leaps through a time portal to 1930 Earth, he does something to change history resulting in the disappearance of the Enterprise. Kirk and Spock soon follow hoping to arrive just before McCoy. They soon find themselves working at the 21st Street Mission for the beautiful Edith Keeler. Spock builds a crude computer and finds two newspaper articles about Edith: one dated 1936 about a meeting she had with President Roosevelt and the other her obituary dated 1930. The question then becomes which of the two are correct. Is Edith Keeler, with whom Kirk has fallen in love, supposed to live or to die?
When a temporarily insane Dr. McCoy accidentally changes history and destroys his time, Kirk and Spock follow him to prevent the disaster, but the price to do so is high.
- The Enterprise passes a planet which sends out shock waves. Sulu is badly hurt at his helm station, and as a pretty Asian yeoman cradles Sulu's head in her arms, McCoy prepares to administer an important drug called cordrazine. "A few drops can save your life." says McCoy. Just then, another shock wave hits, and McCoy accidentally injects the drug into his own arm (the entire hypospray). LOOK OUT!! McCoy has become a madman, and is running around like crazy. He punches a guard, and beams himself down to the planet. What is going to happen next??
Of course, they must try to rescue him. Kirk, Spock, Scott, Uhura and two redshirts (security) beam down after him. When they arrive on the rocky planet, they see a large arched gateway with shimmering rocks. There are rocks all around, and McCoy is hiding behind one. Spock gives him a neck pinch to put him temporarily to sleep. "What is it?" asks Kirk. BIG MISTAKE!!
The arch speaks in a dark, forbidding voice. "A QUESTION!! SINCE BEFORE YOUR SUN BURNED HOT I HAVE AWAITED A QUESTION." The arch then 'says' that it is the Guardian of Forever. It displays scenes from world history within the semi-circle of its arch, ranging from Egypt, to Palestine, to Rome, to modern day, and keeps showing them in a loop. "The Guardian" says they can travel to anywhere in the past they wish. Spock (finally) turns on his tricorder. Uhura is in contact with the ship (to prepare for when they need to beam up).
LOOK OUT!! McCoy wakes up, pushes the two guards aside, jumps into the arch, and vanishes. Uhura says "Captain, I've lost contact with the ship." They all check their communicators, and have nothing but dead air. The Guardian says slowly, "All that you knew is gone." Kirk is confused. Spock theorizes that McCoy must have changed history, when he arrived in the past. Now they must rescue him. Only Kirk and Spock will go to find the solution and rescue McCoy and the Universe. Kirk says "If we fail to correct the damage McCoy did, all of you must jump in. You will be alive at least in the past. Uhura says "Captain, I'm scared". Kirk comforts her. Spock times the 'jump' with his tricorder and they both jump in.
BOOM!! They are back in 1930's Earth in New York, during the Great Depression of the 1930's. Everyone is poor. They steal some earth clothes to 'hide out' and duck into a basment running from a cop. Someone asks from the top of the stairs "Who is there?" They say "We just wanted to get warm." The woman coming down the stairs is beautiful, like an angel. She is a social worker with big ideas, very, very pretty, and Kirk is attracted to her. She says "A lie is not a very good way to say hello." She gives Kirk and Spock a job at 5 cents an hour, and Spock tries to build a computer using copper wire and other cheap material. He says to Kirk "You are asking me to assemble a mini computer using the equivalent of stone knives and bearskins." Spock explains that they cannot determine whether McCoy arrived before or after their 'jump.' But during the time before McCoy comes, Kirk is falling in love with Edith Keeler. Spock finally assmbles enough data to examine newspaper articles from the time to show the two different timelines that would occur, and that Mccoy somehow changed. It turns out that the focal point is Edith Keeler. CAN YOU BELIEVE IT?? In the first timeline, Edith becomesthe head of a peace movement that grows to such size that it stalls the US entry into WWII, but when she did, she unwittingly let Nazi Germany take over the world. But in the other, she is struck by a truck that very week.
Kirk says to Spock "Spock I believe I am falling in love with Edith Keeler. Spock says "Jim, in order for the Universe to be saved, we need to stop McCoy from somehow saving her. Edith Keeler must die as it actually happened."
McCoy lands on the street in the middle of the night, and is stll woozy from the cordrazine. He loses his phaser pistol and a bum picks it up and accidentally phasers himself to disintegration. The next night Kirk is taking Edith to the movies. Edith says "I have been dying to see the latest Clark Gable movie. "Who?" asks Kirk. Edith says "You know Doctor McCoy said the same thing today." Kirk says "Who?? Where?" "He's in the mission, she says. "Wait here" says Kirk. He runs across the street to the Mission. McCoy runs out and Spock too. They're reunited.
Then Edith starts to cross the street. Kirk looks and sees a big truck coming. He begins to go after her. Spock says "No, Jim." Then McCoy jumps toward Edith, and Kirk grabs him, and holds him back. BOOM!! The truck kills Edith. She is dead. McCoy looks with horror at kirk. "Do you realize what you just did, Jim?" Spock says calmly, "He knows, Doctor, he knows." They are returned by the Guardian to the planet. The universe survives. Kirk flips open his communicator. "Let's get the hell out of here." Wow. All is resolved.